A considerable amount of polarization dispersion is accumulated for a signal light which is transmitted through an optical fiber, optical parts such as an optical isolator, etc. over a long distance in an optical communication system. The polarization dispersion occurs due to propagation delay time difference between two principal polarization modes (two intrinsic axes) orthogonal to each other in an optical fiber. When the polarization dispersion occurs, wave-shapes of received signals are deteriorated to lower receiving sensitivity.
As proposed in the Japanese Patent Kokai No. 5-71399 by the inventor, first and second polarization controlling devices are provided, wherein a signal light frequency-modulated by a frequency of f.sub.0 is supplied to the first polarization controlling devices to be transmitted through an optical fiber provided between the first and second polarization controlling device, and the transmitted signal light is supplied to the second polarization controlling device, from which the signal light is supplied to a polarization separating device, in which two orthogonal polarizations are obtained. One of the two orthogonal polarizations is converted to an electric signal, from which a frequency component of f.sub.0 is detected. Then, the first polarization controlling device is controlled, so that the frequency component is minimum in amplitude, and the second polarization controlling device is controlled, so that the frequency component is maximum in amplitude. Thus, a polarization state of the signal light is coincident to an intrinsic axis to suppress the polarization dispersion. Consequently, the deterioration of the received signal is avoided.
Further, it is not negligible in an optical repeating system, in which signal lights are amplified to be transmitted through a long distance of optical fibers, that power losses are caused by polarization dependency of optical parts, and gains of optical amplifiers are influenced by polarization dependency thereof (also defined "polarization haul-burning"). In order to suppress the influence of the polarization dependency, it is proposed in "Observation of new polarization dependence effect in long haul optically amplified system" on pages 25 to 27 of OFC/IOOC'93 (San Jose, U.S.A.), PD5 by M. G. Taylor that modulating polarization of a signal light is effective. For this purpose, it is proposed in "Polarization-insensitive coherent lightwave system using wide-deviation FSK and data-induced polarization switching" on pages 358 to 360 of the Electronics Letters, 17th March 1988, Vol. 24, No. 6 by L. J. Cimini et al that a signal light is frequency-modulated to be transmitted through a polarization maintaining optical fiber to provide polarization modulation which is dependent on a light frequency. In the proposed system, no specific optical device such as an optical phase modulator is advantageously required to realize the polarization modulation.
In the conventional apparatus as proposed in the Japanese Patent Application No. 5-71399, however, there is a disadvantage in that the frequency-modulation unit and the polarization controlling devices are provided at different locations to cause a control signal delayed. This results in the limitation on control response speed.
In the conventional apparatus using the polarization maintaining optical fiber as proposed in the Electronics Letters, there is a disadvantage in that the deterioration of wave-shape is caused by the polarization dispersion of the polarization maintaining optical fiber, when the rate of data is fast to be more than 10 Gb/s.